Direct Transfer of Electrons

Redox reactions can proceed by direct transfer of electrons between chemical species. Examples include the rusting of iron and the metabolic breakdown of carbohydrates. Redox processes also can take place by indirect electron transfer from one chemical species to another via an electrical circuit. When a chemical reaction is coupled with electron flow through a circuit, the process is electrochemical. Flashlight batteries and aluminum smelters involve electrochemical processes. 

In addition to redox reactions due to the direct transfer of electrons and holes via the conduction and valence bands, the transfer of redox electrons and holes via the surface states may also proceed at semiconductor electrodes on which surface states exist as shown in Fig. 8-31. Such transfer of redox electrons or holes involves the transition of electrons or holes between the conduction or valence band and the surface states, which can be either an exothermic or endothermic process occurring between two different energy levels. This transition of electrons or holes is followed by the transfer of electrons or holes across the interface of electrodes, which is an adiabatic process taking place at the same electron level between the surface states and the redox particles. 

We now consider a cathodic transfer of electrons from the conduction band of electrode to the vacant redox electron level in a hydrated oxidant particle to form a hydrated reductant particle in solution OX , + ecB- RED . Equation 8-72 expresses this reaction current, due to the direct transfer of electrons from the conduction band to the oxidant particle based on Eqn. 8-61 as follows ... 

The direct transfer of electrons from the frontier orbital of hydrated hydrogen molecules to the frontier orbital of hydrated o Q n molecules does not take place because its activation energy is high but the indirect transfer of electrons via both the electron level of metallic electrodes and the redox electron level of adsorbed reaction intermediates proceeds at an appreciable rate on metal electrodes. 

A more formal MO treatment of this process suggests that hyperconjugation can be viewed as a direct transfer of electron spin from the carbon p orbital to an s orbital centered on hydrogen (Fig. 29.8) the efficiency of transfer depends on the position of the so-called p-proton relative to the p orbital of the adjacent trigonal carbon. An equation analogous to Equation 29.14 describes the coupling of p-protons,... 

In a similar way in which the oxidation of the bivalent iron cations to the trivalent is explained by the direct transfer of electrons from the solution to the electrode, also the oxidation of some anions can be explained by this simple reaction mechanism, e. g. 

Although it is usually referred to electrochemical reduction of C02, it is preferable to discuss electrocatalytic reduction because it is a catalytic process involving reduction through the direct transfer of electrons more than an electrochemical process only. The same is also valid for the water oxidation step, but there is confusion in literature about these aspects and it is not clear whether they are electrocatalytic or electrochemical processes. 

Oxidation and reduction reactions can take place either at the electrodes, which supply electrons and take up electrons, or by direct contact of atoms or molecules, with direct transfer of electrons. Thus... 

Initial evidence for the intermediacy of surface states came from dark current measurements on n-Ti02 and n-SrXi03 in the presence of oxidizing agents such as [Fe(CN)6] ", Fe +, and [IrCle] [177, 178]. Similar evidence that the charge-transfer process was more complex than direct transfer of electrons from the semiconductor CB also came early from AC impedance spectroscopy measurements on n-ZnO, n-CdS and n-CdSe in contact with [Fe(CN)6] species [179, 180],... 

HMPA Cyclic voltammetry. Cathodic current at the initial Direct transfer of electron from metal 1973... 

An alternative to the application of mediators is the direct transfer of electrons between the prosthetic group of the enzyme and the amperometric electrode (Fig. 19). In this heterogenous reaction the electrode acts as an electron transferase. 

Unlike the oxidation of glucose by oxygen (as in a fire), most biological oxidations do not involve direct transfer of electrons from a substrate directly to oxygen. Instead, a series of coupled oxidation-reduction reactions occurs, with the electrons passed to intermediate electron carriers such as NAD+ before they are finally transferred to oxygen. 

Two types of processes can conduct currents across an electrode-solution interface. One type involves a direct transfer of electrons via an oxidation reaction at one electrode and a reduction reaction at the other. Processes of this type are called faradaic processes because they are governed by Faraday s law, which states that the amount of chemical reaction that occurs at an electrode is proportional to the current, called a faradaic current. 

In photosynthesis, the chlorophylls and pheophytins (close cousins of metalloporphyrins and porphyrins, respectively) play key roles in adsorbing light energy over a wide spectral range and converting it into the highly directional transfer of electrons. It is a marvelous but highly complex process... 

Further studies [67] of enzyme/polypyrrole systems have focused on modification of the enzyme. It was found that the redox dye, Meldola blue, forms a strong complex with alcohol dehydrogenase. It is also known that this dye makes the electrochemical regeneration of the coenzyme NADH possible [68,69]. By electropolymerizing pyrrole, Meldola blue, alcohol dehydrogenase and NAD a membrane was prepared that oxidized ethanol apparently by a direct transfer of electrons to the electrode. 

Such lECMEs (immobilized enzyme chemically modified electrodes) can be used both as potentiometric and amperometric sensors [136]. The covalent bonding of enzyme causes the optimal orientation of electroactive enzyme centers toward the electrode surface. Direct transfer of electrons between bound enzyme and the carbon of an lECME was proved [137, 138]. The large active surface and very thin enzyme layer of lECMEs are the reasons for higher sensitivity, lower detection limit, broader linear concentration range and faster response than in the case of other enzyme sensors. 

Early in these studies it was suspected that the currents developed by metabolishing bacterial systems might be due to one of several factors. The generated current could be, in part, the result of the oxidation of a microbial secretion product, or products the direct transfer of electrons from microorganisms to the electrode surface or both. The substrate itself was not considered to be responsible for current production, as in the absence of the bacteria essentially no current was produced. In order to define the source of the electrons available to the electrode, experiments were performed in which the bacteria were prevented from coming in contact with the electrode (22). This forestallment was accomplished by enclosing the anode in a dialysis membrane. 

We see that oxidation can be described as de-electronation, and reduction as electronation. In ordinary oxidation-reduction reactions the two processes take place simultaneously, sometimes by direct transfer of electrons from the atoms that are oxidized to those that are reduced. 

Direct transfer of electrons from sodium atoms to chlorine molecules or atoms in the reacting systems results in the formation of [Na Cl ] ion pairs. 

Another OCV loss is caused by the crossover of fuel through the electrolyte. Ideally the electrolyte allows the transport of only ions. In reality, however, some fuel permeates across the membrane from the anode to the cathode. In addition, some direct transfer of electrons across the membranes can occur and cause electronic short. A fuel loss due to crossover leads to a current loss. The current loss associated with an electrical short is generally small (ca. few milli-amperes) relative to the typieal operating current of a fuel cell, and therefore is not a significant source of current inefficiency. However, these effects have a significant effect on the OCV of the cell. This is particularly true of a low-temperature cell, in which activation losses are considerable [126]. 

Horseradish peroxidase (HRP) is the most commonly used peroxidase for diagnostic testing (Table I). Other peroxidases (Table II) are used less frequently because they are less easily available or cost more. Tables I and II show the detection schemes vary in their method of immobilization of mediator and enzyme. At one extreme one finds systems based on the direct transfer of electrons from the electrode surface through surface bound mediators to HRP redox centers contacting the surface. At the other extreme one finds systems with freely diffusing mediators and enzyme. 

Cameron and Aust have examined the electron-transfer processes between the two domains using EPR and stopped-flow spectroscopy. FAD was found to be the direct electron donor for the transfer of electrons to all substrates tested, including cytochrome c. A comphcated interaction was found to exist between the FAD and heme cofactors. The addition of electron acceptors was shown to increase the rate of flavin oxidation and the electron-transfer rate between the flavin and heme, although the heme itself was not involved in the direct transfer of electrons to substrate. 

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